The animal(s) within us: life on Earth took many forms before the emergence of mammals and mankind.

• Author: Nusslein-Volhard, Christiane
• Position: Science & Technology

[ILLUSTRATION OMITTED]

ANIMALS DISPLAY an extreme diversity, of form, lifestyle, and body organization. Their ancestors were built simpler than the species existing today. One of the most interesting questions in biology is how these forms evolved over time and which innovations of their body plans helped them adapt to new conditions. The answer is difficult to come by because, in most cases, neither the intermediate forms nor the common ancestors exist anymore. Fossils, which are the only clue to the animals of the past, are very rare and tell us almost nothing about the embryonic development or the genes of these animals.

Animals can be classified based on their similarities and thus their evolutionary relationships into groups known as taxa. Members of a particular taxon all share a common ancestor. The largest taxa are called phyla, and there are approximately 30 of them today, though many more probably existed in the past. The five phyla that include the highest number of animal species are the nematodes, or round worms; annelids, or ringed worms; mollusks; the arthropods; and chordates. These phyla, in turn, are divided into classes and each class then is subdivided into orders, families, genera, and species. For instance, the largest class of the chordates is the vertebrates. The vertebrates themselves are divided into the five orders of fish. amphibians, reptiles, birds, and mammals. The mammals include Homo sapiens--the humans.

The relationship among different species can be recognized more easily in embryos than in adults because, during embryonic development, an animal's basic construction plan becomes apparent. This body plan is displayed most clearly at a stage when the animal is not yet fully developed, and when it is not yet able to feed itself. At this early stage, the body still is comparatively simple, because the structures necessary for adaptation to a specific lifestyle have not yet developed. Therefore, animals often are grouped as similar based on embryonic structures rather than on adult ones. The vertebrates, for example, are subdivided into amniotes (reptiles, birds, and mammals) and anamniotes (fishes and amphibians) based on whether they possess an amnion--a protective embryonic coveting. The amnion, however, is not the only criteria for this classification, as there are insects that form an amnion. The fact that some vertebrates have an amnion is based on homology. in other words, derivation from a common ancestor, while the presence of an amnion in vertebrates and insects is based on analogy, namely similar function that has arisen independently during the evolution of the two phyla.

In many cases, it is difficult to determine whether the animal lacking a particular trait is more basic and its ancestor preceded the one of the animal that has it, or whether the trait had been present originally but subsequently was lost in evolution. Because of these uncertainties, the determination of evolutionary relationships among animals always has to rest on several criteria.

Nowadays, the most reliable criteria no longer are morphological, but molecular characteristics--for instance, number of base exchanges in homologous genes. One reliable approach of molecular phylogeny compares DNA sequences of genes that are present in all animals--in particular, gene segments that do not code for proteins. In such noncoding DNA segments, changes are assumed mainly to be accidental, without an influence on the phenotype and therefore not subject to selection. Once a mutation in noncoding DNA has occurred, it therefore should be passed on to all descendants. At present, variation in the genes encoding ribosomal RNA is the most commonly used criteria for the taxonomic classification of more distantly related species. For the analysis of the differences among more closely related species, and the variation among individuals within them, mitochondrial DNA sequences are employed. Mitochondrial DNA mutates more often than chromosomal DNA, as there are fewer DNA repair mechanisms operating in mitochondria. So, there will be more informative sequence differences occurring in the comparatively short lime after the separation of two closely related species.

When genes among different organisms are compared, one striking observation is how similar they are in sequence and, often, in molecular function. This especially is surprising in the case of developmental genes that regulate how body plans and organs are formed. These genes sometimes are so similar that they can replace each other readily in two animals that look very different. This suggests that all animals arose from one ancestor that already had a set of developmental pathways and a rather detailed body plan, with top and bottom, front and rear, and several organs at specific places. Some such genes even can be traced back further to the single-celled ancestors of animals, which, in turn, are presumed to have their origin in some kind of bacteria-like cell. This is evident from the observation that the elements of the basic metabolic and genetic machinery of a cell are common to organisms with evolutionary paths that separated billions of years ago, such as humans and bacteria.

Bacteria-like cells probably were the first organisms on Earth. Bacteria are relatively simple cells surrounded by rigid cell walls that determine their shape. Bacteria already feature the basic mechanisms for cell replication, such as DNA, RNA, protein synthesis, and ribosomes. They do not yet have a nucleus and, their DNA, a ring-shaped molecule, is arranged loosely in the cytoplasm. While there is no true sex in bacteria, a form of genetic exchange does lake place between individual cells. In fact, DNA sometimes can be transferred among cells of different bacterial species. This phenomenon is known as horizontal gene transfer. Bacteria do not display a particular diversity of shapes and, although some species form aggregates of cells, there are no truly multicellular species. However, they do possess a remarkably diverse biochemical ability to convert and build materials of all kinds, allowing for adaptation to even the most extreme conditions of life; for instance there are bacteria that grow best at 230[degrees]F.

The first organisms containing a cell nucleus, the eukaryotes, are assumed to...